Carry-out and/or delivered fast food continues to enjoy widespread success due to the convenience associated with foods which are prepared in restaurants and may be enjoyed in the comfort of the customer's own home. Conventionally, food products such as pizzas are placed in a carton or box for transport to the home or office for consumption. Particularly with delivered fast foods, the food container may also be placed in an insulating pouch to prevent cooling of the food. However, even an insulating pouch is often insufficient to prevent cooling where the customer or delivery person must transport the food a substantial distance.
Where a delivery person is making a number of deliveries in a single delivery run, the problem can be particularly apparent due to the elapsed time in making successive deliveries, especially where certain addresses are difficult to find. Often a large number of pizzas can become cold. When a customer realizes the pizza is cold, the delivery person has usually left, making it difficult for the customer to complain of the cold food, and the customer is apt to select another establishment for their next delivered meal. To avoid cooling of the food, the delivery runs may be limited to one or two deliveries; however, this reduces the efficiency of the delivery operation since additional trips must be made to the restaurant to pick up hot orders.
To solve the above problems, it has been proposed to provide heating devices which may be transported with the prepared food to insure arrival in acceptably heated condition. However, previous attempts have not achieved a heating device which is sufficiently reliable and inexpensive to manufacture.
For example, U.S. Pat. No. 3,721,803 to DiStefano discloses a pizza pie warming carrier in which a plurality of electric resistance wires are sandwiched between fabric coverings of an outer carrier such that a boxed pizza may be placed in the carrier and the boxed pizza is kept warm by the heat generated as electricity is passed through the wires. However, manufacture of such a device can be expensive due, in part, to the difficulty in properly positioning the wires within the carrier. In view of the expense, such a device is only cost effective for use as a reusable carrier for holding pizza boxes therein. In addition, in the DiStefano arrangement, portions of the carrier adjacent the heating wires may become overheated, while areas between the wires may not be sufficiently heated. Thus, a further difficulty with the DiStefano arrangement is in providing an evenly distributed heated surface such that sufficient heat may be transferred, without risking localized overheated portions.
U.S. Pat. No. 2,971,073 to Eisler discloses an electric surface heating device in which a metal foil forms a resistance heating device as electricity is passed therethrough. In the '073 patent, a circuit pattern is formed in the metal foil by placing a stencil over the metal foil and utilizing an abrasive brush to strip away portions of the metal foil leaving a zig-zag or meandering circuit pattern which forms the heating element. However, due to the somewhat crude nature in removing the foil to form the circuit pattern, a large amount of foil is removed, thereby reducing the effectiveness of the heating element, and also leaving large areas unheated. The '073 Eisler device is thus unsuitable for use as a heaterboard in warming food products, since it is difficult to properly distribute the heat in the heating device. When heating or maintaining warmness of a pizza, the heated surface aids in maintaining crispness of the pizza crust. Where large areas of a heating device are unheated, soggy crust portions can develop due to the grease and moisture associated with the pizza. Moreover, since a large amount of the foil is removed, the effective area of the heating device is reduced and the power per unit area of the heater which is transferred in the form of heat is also reduced. The Eisler heating device is disclosed as particularly suitable for use in heated wallpaper arrangements and would not appear desirable as a food warming device.
U.S. Pat. Nos. 3,020,378 and 3,483,358 to Eisler disclose metal foil heating elements for use in packages in which the metal foil is cut to form a meandering circuit path in the foil prior to lamination to a substrate. After cutting, the foil is deformed to change the surface dimensions of the foil to open up the cuts to define the electric circuit; however, due to the thinness of the foil, the foil can be broken during deformation and, without the deformation, the cuts may close up forming short circuits in the circuit pattern. Moreover, when the foil is cut, handling is more difficult since the foil is even more fragile after cutting and more subject to deformation as a result of forces incurred in feeding the foil. Since the foil is more difficult to handle, it is also more difficult to place the foil on a substrate in proper registration, such that the cut portions do not contact to form short circuits.
In U.S. Pat. No. 3,026,234 to Eisler, a meandering circuit path is cut in a metal foil and an insulating layer is interleaved or interwoven with the cut foil to insure insulation between adjacent cut portions, thereby preventing short circuiting of adjacent cut portions. However, as in the above Eisler devices, handling of the thin cut foil is difficult and thus placement of an insulating sheet such that it is interwoven with the cut foil is difficult, and risks breaking or tearing of the foil which could create an open circuit rendering the heating device inoperative.
Thus, a heating device which is reliable in providing properly distributed heat and which is inexpensive to manufacture is needed.